With the rapid advancement in development of CAD/CAM systems, most of which are vector display systems, color considerations have made it important to develop a color raster scan display system which emulates a vector display system. Yet until now, vectors displayed on raster display systems lacked some of the attributes of those displayed on vector display systems.
For example, a typical raster scan display system includes a pixel (picture element) memory which is a matrix representation of what is being displayed on the screen. Each pixel memory location corresponds with one location on the screen (e.g., the pixel memory contains one register corresponding with each of the 1,048,576 pixels of the 1024.times.1024 raster CRT display). The raster system takes vector information in the form of end points and transforms these end points into the appropriate registers of the pixel memory. As each vector is written into the pixel memory, the appropriate pixel color is placed into the corresponding register in the memory matrix. After a new vector is written over an old vector, the affected pixel memory registers contain the color of the new vector; not that of the old vector. Thus, in a conventional raster display system (such as IBM's Model 5080, the system sold by Spectagraphics of California, and the Vistagraphics systems of Sanders Associates), the new vector completely overlaps the old vector; one cannot determine that the old vector ever existed, and it is not possible visually to distinguish either the existence or extent of overlap. By way of contrast, on a vector display system, one can determine the existence of the overlapping vectors by observing the increase in intensity which results when the electron beam twice sweeps over any points of overlap.
Although it was plainly desirable to make visually obvious which vectors overlap and the extent of such overlap, most conventional raster display systems (including the IBM, Spectragraphics and Sanders systems referred to above, a wide range of manuals and other documentation for which is readily available to those working in the field) did not do so. The principal reason they did not was the problem of speed. To determine whether to highlight (by increase in intensity or change in color) a pixel, the conventional approach was for the central processing unit of the system to read the existing data from memory, examine the data to determine whether to write the highlight color or the vector color, and then write the appropriate data back into the memory. This procedure was far too slow to be satisfactory. It had to be accomplished on a pixel by pixel basis until all the pixels in the vector were written; and this typically took from 1.5 microseconds to 10.5 microseconds per pixel, a very slow rate. Such a slowing of response time decreased the interactiveness of the system and caused low performance and productivity.